A coil spring biasing each of the intake/exhaust valves of an internal combustion engine may cause surging, i.e., self-excited vibration during the high-speed rotation of the engine to result in the abnormal motion of the valve in some cases. The following technology is disclosed as a means for preventing surging (Japanese Patent Laid-open No. Hei 8-177422 (FIGS. 1 and 2)). The coil spring is partially formed with a tight-winding portion. Mutual-contact portions between the spring members of the tight-winding portion are designed to contact each other through corresponding flat surfaces generally perpendicular to the axial direction of the coil spring.
In this technology, at the normal rotation speed of the engine, the spring members of the tight-winding portion of the coil spring can be maintained in a mutual-contact state. However, a portion other than the tight-winding portion performs the normal expansion-contraction operation of the coil spring. The frequency of compulsory vibration applied to the coil spring via a cam reaches the vicinity of the resonance point of the coil spring. At this time, the tight-winding portion starts contact and separation due to the self-excited vibration and the confronting flat surfaces of the tight-winding portion collide with each other to lose the energy of the self-excited vibration. Further, the confronting flat surfaces of the tight-winding portion displace the oil remaining on the periphery of the tight-winding portion, so that the coil spring undergoes damping force. These suppress the surging of the coil spring.
One of Transactions of the Japan Society of Mechanical Engineers discloses a method of reducing the vibration transmission of a bearing portion by use of the damping effect of a squeeze film damper (Transactions of the Japan Society of Mechanical Engineers, (Series C), Nol. 53, No. 495 (1987-11), thesis No. 87-0049A, “Study on dumping effect of squeeze film damper”).